document that SetCurrent() must be called on a visible window; assert that this is the case

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@34454 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/glcanvas.cpp

@@ -243,11 +243,6 @@ void wxGLContext::SetCurrent()
   {
     wglMakeCurrent((HDC) m_hDC, m_glContext);
   }
-
-  /*
-  setupPixelFormat(hDC);
-  setupPalette(hDC);
-  */
 }
 
 void wxGLContext::SetColour(const wxChar *colour)
@@ -586,6 +581,12 @@ void wxGLCanvas::OnSize(wxSizeEvent& WXUNUSED(event))
 
 void wxGLCanvas::SetCurrent()
 {
+  // although on MSW it works even if the window is still hidden, it doesn't
+  // under wxGTK and documentation mentions that SetCurrent() can only be
+  // called for a shown window, so check it
+  wxASSERT_MSG( GetParent()->IsShown(),
+                    _T("can't make hidden GL canvas current") );
+
   if (m_glContext)
   {
     m_glContext->SetCurrent();